How to get the chi2 from roofit?

I use roofit to do a 2D binned ML fit.

RooFitResult* r=model.fitTo(*data,Save(kTRUE));

Then I want to get the chi2 . I use the following codes:

RooChi2Var chi2("chi2", "chi2", model, RooDataHist("data","data",RooArgSet(M,Q),hmqrs));
Double_t chi2_val = chi2.getVal();
cout << "chi2 from RooChi2Var = " << chi2_val/(hmqrs->GetNbinsX()*hmqrs->GetNbinsY()-2.0) << endl;

I get the error information:

   [#0] ERROR:Eval -- RooChi2Var::RooChi2Var(chi2) INFINITY ERROR: bin 0 has zero error, but function is not zero (0.000535416)
chi2 from RooChi2Var = 0

How can I solve this problem? Or are there any other good methods to get chi2.